Refrigerant and desiccant dehumidifiers are the most common means used to remove moisture and humidity from water-damaged residential and commercial buildings. They are “closed” systems in that the building's air is continuously recycled through the dehumidifier and no outside air is introduced to the process. Dehumidifiers remove moisture from the air and lower the relative humidity which speeds the evaporation process. Dehumidification systems have a number of shortcomings. The time taken to process a wet building's air for lowering the relative humidity levels to acceptable levels for drying to begin can be in excess of 24 hours. Because this air is recycled, unpleasant odors are slow to dissipate. Mold spores and other air contaminates are not removed and risk being spread throughout the building. Dehumidifiers have a very limited temperature operating range and perform poorly below 50° F. and above 85° F. Humidifiers are usually operated at normal building temperature levels of 72° F., a temperature level which is also conducive to mold growth. Still yet another problem associated with the use of dehumidifiers is their consumption of large amounts of electrical power.
Recently, techniques utilizing heat to dry water-damaged structures have been developed. One type of system is comprised of a boiler, heat transfer fluid, and heat exchangers. The boiler, located outside the building, heats a fluid which is pumped through hoses to heat exchangers located in the structure. Heat exchanger fans blow room air through the heat exchanger which warms the air and lowers the relative humidity. The heat and lowered relative humidity accelerate the evaporation process. Exhaust fans remove the hot, moist air from the structure. The volume of air exhausted and replaced with fresh, outside air is sometimes controlled by a humidity sensor.
A second type of system uses hot air as the heat exchange medium. Located outside the structure being dried, fresh air is drawn into a trailer-mounted furnace, heated and reduced in relative humidity, and then blown into the water damaged structure. The hot, dry air heats water molecules by convection and accelerates evaporation. An exhaust fan removes the warm, moist air and exhausts it to atmosphere. Because fresh, outside air is used to replace the building's air, hot air dries are considered “open” systems.
“Open” hot air systems offer a number of advantages over dehumidification. By displacing the building's moist air rather than dehumidifying the air, the relative humidity level in the building can be reduced to below 40% within an hour or two and drying can begin. The introduction of fresh air removes odors associated with dank, wet air. Heat is especially effective at drying contents such as fabrics, books, and furniture. A rule of thumb says for every 10° C. temperature rise, the evaporation rate is doubled. Open hot air systems typically raise building temperatures by 15° to 20° C. over the standard 72° F. Wet buildings are always at a risk of developing mold problems. Hot air system drying temperatures are well above the 50° to 80° F. range for mold growth.
While effective drying tools, as developed, open hot air systems are not without weaknesses. Open systems require a balanced air flow into and out of the building in a managed circulation pattern for optimal performance, but the systems have no means to control air flow. The supply and exhaust blowers are located within the drying trailer, and lengthy runs of flexible duct are required to deliver fresh hot air and remove moist air from the building. Besides being inconvenient to install, lengthy runs of flexible duct greatly reduce air volumes thereby putting the system out of balance. Differing lengths of hose and the route of the hoses put differing static pressure loads on the blowers for which they do not compensate. Also, the trailer location sometimes makes optimal exhaust duct positioning impossible.
The very nature of “open” drying systems makes achieving high levels of thermal efficiency problematic. There are but two temperature sensors controlling heat output of the furnace and no means to measure or automatically control air flow volumes. The temperature sensors are both located within the trailer, not in the structure being dried. One sensor is placed in the hot air stream exiting the furnace and one is in the building exhaust air stream entering the trailer. The furnace sensor signal is used for controlling the furnace's heat output to an operator-selected set point. The exhaust stream temperature sensor is used to prevent overheating of the structure. A high limit set point is operator-selected and an exhaust duct signal at the limit will override the furnace output temperature control. However, because the exhaust air cools as it travels through the flexible duct, especially once outside the building, the exhaust air temperature entering the trailer is considerably lower than the actual building temperature.
The lack of air flow controls also contributes to “open” air drying system inefficiencies. These systems typically operate at a constant air flow volume with equal amounts of air being introduced into the building and being exhausted. As a water-damaged structure dries, the volume of moisture evaporating declines and the relative humidity of the air being exhausted from the building likewise declines. Consequently, low humidity air along with a great deal of heat energy is often exhausted to atmosphere.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.